This invention relates to the treatment of volcanic igneous rocks, commonly called volcanic cinders, to recover the gold, silver and platinum group metals therefrom.
A common volcanic ore is composed of silicates and oxides with major amounts of iron, magnesium and aluminum, with lesser, but significant, amounts of sodium, boron, calcium, fluorine, and phosphorus (from tenths of a percent to three percent of these elements). The ore contains a major amount of magnetite. At a grind of 43 percent minus 65 mesh plus 200 mesh and 57 percent minus 200 mesh, 54.6 percent of the sample was strongly magnetic. Not all of this magnetic material is magnetite, however, since much of the magnetite is locked with non-magnetics. The phosphors and fluorine are assumed to come from apatite, Ca.sub.5 F(PO.sub.4).sub.3, a mineral commonly associated with magnetite in igneous rocks.
Volcanic cinders, as typified by the ore described above, are extremely resistant to all of the procedures normally used in the treatment of precious metal ores. A typical smelting process is one in which all of the materials charged to a smelting furnace are completely fused or melted, resulting in two or more liquid products which stratify into separate layers upon sending, with the slag, the liquid of lowest specific gravity, forming the top layer which is skimmed off. In most smelting operations the slag is a waste product which serves as a vehicle to eliminate substances which are not desired in the valuable products recovered from the lower layers of molten material. The bosom layer can be a metal, such as lead in lead smelting, iron in iron smelting, or copper in some types of copper smelting. An intermediate layer can be a matte (principally molten sulfides), or a speiss (molten arsenides and/or antimonides). It is well known that molten lead serves as a solvent, or collector, for gold, silver, and the platinum group metals in the smelting of lead ores. Similarly, liquid copper and liquid mattes (molten sulfides of copper and iron, or nickel and iron) serve as collectors for the precious metals (gold, silver, and the platinum group) in copper and nickel smelters.